Desktop Tower Defense and World of Warcraft aren't similar in any way whatsoever; not only do they belong to completely different genres, but they're played by completely different types of gamers. And yet somehow, Dungeon Defenders manages to combine the two into a deep, inventive, and addictive experience that retails for a mere $15. If the idea of setting up towers, releasing waves of enemies, and then helping the towers kill everything in sight appeals to you in the slightest, Dungeon Defenders is a must-buy—especially if you also enjoy playing co-op, or don't mind a significant challenge.

Like most of the new action-oriented tower-defense games (think Rock of Ages), Dungeon Defenders divides its gameplay into two phases. The first is a setup phase, but instead of playing God, you play a character in the third person, running around the map and building towers and other defensive structures. Signs tell you how many enemies will come out of each of the arena's doors, and the goal is to keep the creeps away from your crystal; the crystal can withstand some damage, but you lose if it's destroyed. You can also heal yourself and repair towers that have become damaged.

In the second phase, the enemies are released, and all hell breaks loose. Unlike in most tower defense games, in Dungeon Defenders you can attack enemies personally, instead of just messing with your towers as a swarm of bad guys march on them. This fact creates countless risk/reward scenarios. With a huge wave of enemies bearing down on you, you have to decide whether to tend to your towers or jump into the fight, and the wrong decision can mean the difference between life and death.

Underlying all of this are some World of Warcraft-style RPG mechanics. For starters, you can choose from among four character classes whenever you create a hero, and you can switch heroes between each wave. The easiest character to use is the Apprentice, who can cast offensive spells from a distance, has a ground-pound attack, and erects towers that fire magical energy at the bad guys. Next up is the Squire, who attacks with his sword and creates medieval-style traps. The third is the Huntress, who attacks with a crossbow, moves stealthily, and sets traps. And finally there's the Monk, who fights with a spear and can heal his co-op partners. (More character classes are on the way via DLC.)

As you play through the various maps, you level up, unlocking new towers to build and earning points that you can spend to improve your character. The points can make a big difference depending on your play style. For example, you might want to spend them making your towers more powerful, and then just use your character to keep the towers in good repair while enemies attack them. Or, you might use your points to create a powerhouse of a character, and then kill as many enemies as you can in person. You'll also find some gear to wear after each wave you defeat; the game conveniently tells you with a thumbs-up or a thumbs-down whether it's better than what you already have. (You can sell unnecessary gear, or put it in your item box so that another one of your heroes can use it.) You can even buy pets and bring them along to help with your adventures.

The single-player campaign is meaty, with 13 missions and replayability galore thanks to the fact that you can try it with a different hero or play style. You can also raise the difficulty once you get the hang of it (you'll definitely want to start with Easy if you're going solo). And that's not all: You can play a horde mode, a challenge mode with bizarre rules (for example, the crystal you need to defend changes position every 30 seconds), and a pure tower-defense mode that removes the action elements.

However, it can be exhausting trying to handle the huge, difficult levels by yourself, so Dungeon Defenders' deep and rewarding co-op modes offer the better experience. You can play locally or online with up to three friends, and the entire strategy of the game shifts when different character classes are battling the same waves of baddies. To win, you'll each have to focus on what your class does best.